There is a broad range of commercially available automatically operated portable devices that allow to perform an “objective” assessment of the auditory sensitiveness through the recording and processing of temporary physiological responses caused by short acoustic stimuli. These devices commonly use the Auditory Evoked Potentials of the Brainstem (AEPBT), which are known from Jewett D L, Romano M N, Wilson J S. (Human auditory evoked potentials: Possible brainstem components detected on the scalp. Science 1970; 167:1517-8) and/or oto-acoustic emissions generated at the auditory recipient (OAE), which have been described by Kemp D. (Stimulated acoustic emissions from within the human auditory system. J Acoust Soc Am. 1978; 64:1386-9).
However, these devices and methodologies still have shortcomings that must be solved. A very important problem that still has to be solved is the high rate of “false positives” found during the initial auditory examination of neonates, which is carried out when the neonate stays at the maternity hospital. Many of these neonates (classified as false positives) show temporary hearing losses, caused by problems encountered by the transmission of sound through the structures of the middle ear, and not by a neuronal damage of the recipient (inner ear), which is the main object of the screening tests. Temporary conductive losses in neonates (caused by serum fluid of the middle ear) are spontaneously solved in a few days. However, it is necessary to perform a second test, which generates anxiety among parents, makes the follow-up more complicated, as a result of the large number of cases involved, and has even raised doubts about the implementation of the universal screening programmes.
In order to solve this problem and to make a difference between both types of disorders (conductive and sensorineural losses) it does not suffice to establish the auditory sensitivity vis-á-vis a sound which is presented, in a natural way, through the external acoustic meatus (air conduction mode). In the case of auditory disorders caused by problems in the sound transmission through the outer and middle ear (conductive loss), sound vibrations can be properly perceived if they are directly transmitted to the neural recipient (inner ear) through the bone (bone conduction mode) since, in this case, they do not cross the affected structures of the outer and middle ear. This allows, when evaluating the auditory sensitiveness by means of sounds transmitted through both modes, to discriminate between conductive disorders (where the bone conduction mode yields normal, but air conduction mode does not) and those caused by a permanent damage to the recipient or neural route (where the auditory sensitiveness is also affected, irrespective of the sound transmission route eventually chosen).
The temporary physiological responses to short acoustic stimuli (AEPBT and OAE) show serious limitations in order to assess the bone sensitiveness. It has been acknowledged that the technological process implemented to obtain and identify these responses to the bone conduction mode stimulation is more complicated, requires a great deal of experience on the part of the person in charge of the evaluation, in order to appropriately identify the threshold response, and there are no effective automation procedures, which is unavoidable in the case of a Universal Neonatal Screening.
Steady state auditory potentials (SSEAP), (known from Cohen L T, Richards F M; Clark G M. A comparison of steady state evoked potentials in awake and sleeping humans J. Acousti. Soc. Am. 1991) are a valid alternative for the objective assessment of auditory sensitiveness. These responses consist of constant periodical signals that can be caused by means of long tonal stimuli with amplitude and/or frequency modulation (between 70 and 110 Hz). Considering that these SSEAP are generated at the brainstem, they are not affected by sedation or sleep, which facilitates their use with neonates. As a result of its periodical nature, the SSEAP provoked by modulated tones can be better analysed within the frequency domain (Fourier analysis), and they are represented as spectrum peaks limited to the modulation frequency used. This facilitates their automatic detection through different statistical methods calculated within the domain of frequency (Valdés J. L., Pérez-Abalo M. C, Martin V, Savio G, Sierra C, Rodriguez E, Lins O. “Comparison of Statistical Indicators for the Automatic Detection of 80 Hz Auditory Steady Responses”. 18 (1997): 420-429. Sep. 11, 1997). From Lins O G and Picton T W. (Auditory steady-state responses to multiple simultaneous stimuli. Electroencephalogr. Clin. Neurophysiol. 1995; 96: 420-32) it has been also known that multiple SSEAP can be obtained for tonal stimuli that are simultaneously presented, thereby reducing the duration of the hearing test.
As a result of these advantages, several recent patents (U.S. Pat. No. 7,014,613 B2; U.S. Pat. No. 6,778,955 B2; U.S. Pat. No. 6,524,258 B1) propose different alternative methods and apparatuses to facilitate the use of SSEAP for the objective assessment of hearing. Since these responses show a very low amplitude, especially at low ages, and they are disturbed by a higher amount of noise, documents U.S. Pat. No. 7,014,613 B2 and U.S. Pat. No. 6,524,258 B1 claim new types of acoustic stimuli, presented in the air conduction mode to generate responses with a higher amplitude, that can be more easily detected. Document U.S. Pat. No. 7,014,613 B2 also claims a series of methods to render the calculation of the SSEAP more efficient, and proposes to use the weighted averaging method, so that the most noisy records have less weight for the calculation of the multiple SSEAP. However, this method is useful only to reduce the effect of temporary noise contamination (as opposed to the stationary one) and is not appropriate for other sources of noise that re constant along the time (stationary) and that are also present at the SSEAP. On the other hand, the method used to calculate the amplitude of the noise (within the desired range of 70 to 110 Hz) does not allow to accurately ascertain what is happening in the vicinity of each one of the evoked signals or multiple SSEAPs. In this case, the relative weight of a bioelectric record collected, which is adequate for some signals, but not for others, can be proportionally reduced. The patent (U.S. Pat. No. 6,778,955 B2) divulges another method to achieve a more efficient calculation of responses with a low signal-to-noise ratio, which can be applied to temporary responses (OAE) or stable state responses (SSEAP), but also, it only considers those responses that have been generated through sound stimulation presented to the patient by the air way. Therefore, none of these technological and methodological solutions allows to use the multiple SSEAP for a primary hearing test that can discriminate, in a fast and efficient manner, the type of hearing disorder (conductive or sensorineural), if any. In fact, all the automatic hearing screening devices currently existing show the results in a binary format (normal or abnormal audition). Furthermore, no automatic and easy to use SSEAP devices that can be used by a Universal Neonatal Screening Programme are currently commercially available.